I don’t know how old I was when I saw my first comet. It was probably (no bets though) somewhere around ’64 or ’65. There was a comet in the morning sky, and I remember Dad got us out of bed to see it. I don’t know what hour of the morning it was, but they opened the front window drapes to their bedroom and we looked up, and there it was.
This dazzling white intruder with a tail stood stark among the cold morning stars. I marveled at the wonder in the dark.
The ancients had little understanding of these mysterious wanderers among the stars. As far as most were concerned, the heavens didn’t change. The same stars that looked down on their ancestors, looked down on them. The only oddity were the planets which sailed among the stars, and even still they explained those.
But about the time they began to think nothing changed in the skies, along would come a comet. It would blaze out in the sky, its long sweeping tail obscuring the stars. And slowly it would fade away, and things would return to normal until the next one appeared.
There were some interesting exceptions to this school of thought. In 6th Century India, it was felt that certain comets might indeed be the same comets reappearing. The Indian astronomer Bhaṭṭotpala had a list of comets by name and estimates when they’d reappear. Unfortunately, how he arrived at those conclusions isn’t known. This was almost eight hundred years before Edmund Halley said the same thing and proved it.
One thing is certain, most of the ancients couldn’t make heads or tails of these visitors in the sky. Since comets put disorder into an orderly sky, than they couldn’t be good. So, in many cultures, comets came to be seen as something evil. Of a portent of disasters yet to come.
I used this idea in my first novel to a degree. Will is showing his friends the telescope Jewell bought him for Christmas, and he’s talking about a comet that many had high hopes. Comet Kohoutek was touted as being the Comet of the Century, but fizzled. Instead of this dazzling object that might have been visible in daytime, it ended up a third degree magnitude ghost across the stars.
It was an abject lesson in how fickle these visitors from the depths of space could be. What’s thought to have happened is the comet came so close to the sun (they call these kinds of comets, Sun Grazers – Some come so close, they crash into the Sun) that the intense heat cooked most of the dust and ice off the comet. There was also evidence to suggest that the nucleus, a ball of rock and ice had shattered under the heat and gravitational pull of of our local star.
All things considered, it was still a really good comet, and it stood stark against the winter sky.
Will remembers that it was indeed the portent of a disaster for him. Stepping out into the snowy winter night, he saw it, and spent several minutes studying it through the small telescope he had at the time. When he went in to tell his parents and siblings that you could see the comet, they called him stupid and didn’t know what he was looking at.
None of them moved a muscle to go see this marvel in the night sky.
As Will recalls that began a cold winter between him and his family that eventually became as cold as the depths ridden by the comet itself. He was unaware of it at the time, but eventually he’d feel as far from his family as the comet was from the Sun.
But enough of that.
It wasn’t until the 16th century when astronomers like Tycho Brahe and others began seriously considering them as astronomical objects. Tycho and others measured the Parallax of the great comet of 1577 and concluded it was at least four times further from the earth than the Moon at the time of the observation.
Of course it didn’t help to have Galileo reject the study by saying they were nothing more than optical illusions.
Slowly people began to accept that comets were astronomical objects. Issac Newton said they were solid objects, and that they emitted gases or vapors that were heated or ignited by head from the sun.
The great thinker Emmanuel Kant thought of them as made of some volitale substance that vaporizes, and that’s what forms the tail.
It had to wait for the 1950s for Astronomer Fred Whipple to describe a comet as a “Dirty Snowball,” a description that would turn out as not far from the truth.
Comets are composed of what we call a nucleus. If we look at a fully developed comet, it what’s in the middle of that fantastic cloud spewing from it. Far from the sun, a comet might be mistaken for any of a million asteroids. Indeed, the more we study both asteriods and comets, the more the distinction between them blurs. Asteriods have been observed to sport comet like tails on occasion, and then there’s comets which barely develop a tail at all.
What both are is the left overs of creation. When our solar system formed, what was left after the planets formed were left to the drift through space, the dregs of that fantastic event. Many orbit far beyond Pluto in the Orrt and Kepler belt, and have done so for billions of years.
But occasionally, one drops towards the inner solar system. Maybe some gravity tug altered it’s orbit enough for that happen. maybe it collided with another object. Or maybe, as some think, the mysterious, yet to be discovered Planet 9 slingshots them towards the sun.
However it happens, the comet begins to fall towards the sun.
But the Sun is a long ways away, and the fall can take hundreds if not tens of thousands of years. but if there’s one thing the Universe has plenty of, it’s time. Kohoutek began its long trip to the Sun while Humans were still living in caves. All of our history is crammed into the time it left its orbit so far away and the time it blazed forth for me to stand in the snow and look at it.
Eventually, it passes the frontier marked by the orbit of Pluto. By now, the gravity of the Sun has it, and the comet begins to fall faster and faster. It drifts past the orbits of Uranus and Neptune, picking up speed with every passing mile. By the time it passes the orbit of Jupiter and flies through the Asteroid Belt, it’s moving at a respectable speed.
Now things begin to happen to the comet. The warmth of the sun begins to melt ices on the surface. The warmth penetrates the rock and taps the frozen springs of gases inside. Jet’s of gas erupt from the surface. What had been a frozen landscape becomes like a geyser field in Yellowstone Park. The nucleus is suddenly cloud covered in a haze we call the Coma.
As the comet spews it’s gases, the gases in the coma catch the light of the sun, and the light and solar wind push the gases back and way from the comet, forming it’s tail.
And someplace on Earth, some amateur astronomer or some machine notices something that wasn’t there before. It’s dutifully reported and other telescopes turn to find the new visitor.
Now how bright it’s going to get can be almost anyone’s guess. There’s a lot of things to consider such as how big the nucleus is. Most are rather small, measured in football lengths instead of miles.
Then, there’s how much ice is really in it, how close it’s going to come to the sun, and so on. Comets are very finicky.
The biggest surprise to people is that there’s almost always at least one comet in the sky. But they’re often too far and too faint to be seen as much more than a smudge through a telescope.
Especially true of these are the “Short Period Comets.” These are comets that are trapped within the inner reaches of the Solar System. Most never journey much further from the Sun than mighty Jupiter. Trapped this close to the Sun, very few of them sport much in the way of tails, and are slowly running out gas. Eventually, they’ll go dark and be just one more asteroid spinning around the sun.
Then we get the good ones. These are the Halley Comets, the ones that dash in and then head out to beyond Saturn and return like a train on a schedule. Most tend to put on a good show, and then leave and return like clockwork.
The most famous of these is Halley’s Comet, and it seems that humans have seen it often. There’s records of it going all the way back to 240 BC that seem to indicate it’s the same comet (orbital tracks agree with this).
In modern times the 1910 passage of Halley was spectacular indeed, with a close approach of less than 15 million miles to the Earth. Earth even passed through its tail.
The next time around in 1986 wasn’t so good. The Earth and Halley’s Comet were on different sides of the Sun, but the 2061 visit promises to be spectacular. Unfortunately, I’ll be 105 next time around so I’ll miss it.
Then there’s those that belong to the Oort Cloud of comets. Many have never been past our Sun, and those are the ones we pin our hopes on for a true spectacle. But as we’ve seen, we’re sometimes disappointed.
And as we’ve recently learned, occasionally we’re visited by a nomad from the stars. A comet, torn from the system of its birth, wanders the galactic wasteland, and every so often, one comes our way. 2I/Borisov is an example. There are estimates that a lot more interstellar debris than we know of wanders through our system. So far, we’ve only seen two objects from the great beyond go through.
Today, we’ve visited several comets. Back in the mid 80s, as Halley’s Comet came parading through the Solar System, an armada of space probes from Earth were there to greet it.
More recently, the European Space Agency orbited the Rosetta probe around Comet Churyumo/Gerasimenko. A dramatic feat eclipsed only by the hair raising landing of of the Philae lander which gave us close up pictures and reports from the ground.
Much of the mystery of Comets has evaporated away. But as I realized the other morning as I got up early and went to look for Comet NEOWISE, nothing has eclipsed their beauty.